The present invention relates to an image display apparatus and more specifically to a display control apparatus for displaying patterns such as characters and figures on a raster scanning type CRT display.
It is one of the important functions of the display processing apparatus to be able to display on a cathode ray tube (CRT) screen information such as sentences (hereinafter referred to simply as text), figures and other images (simply referred to as graphics) stored in memory (e.g., a dynamic memory).
In recent years, demands for so-called new media related equipment such as personal computers, word processors and data base network services for private or home use have been growing remarkably. The importance of display processing as man-machine interface is therefore not only in the office automation (OA) equipment field but also in the home or consumer-use new-media-related equipment field. Also the mode of display has diversified. At the same time, the processes executed by the microprocessor which controls the system have become complex and large in volume. Under these circumstances, there are increasing demands for display equipment capable of controlling and processing display data efficiently.
With conventional display apparatuses of this kind, display is performed by first storing in a refresh memory information to be displayed such as text and graphics. Then the above information is successively read out in synchronism with the CRT scanning timing and converted into a video signal which is to be supplied to the CRT. For the text display in particular, the CRT screen is orderly divided into a large number of small sections, each of which is assigned with a character. The display position of the character is related to the address of the character code in the refresh memory. A series of character patterns obtained by reading the character code in successive addresses is used to display the text on the screen.
It is also known to display characters with underline or in a blinking manner. In order to perform the above modified display, pattern modifying information (hereinafter referred to as attribute information) specifying the shape, color and blink mode for each display datum is stored in the refresh memory with the character code of the display data. Upon display, the attribute information is read out in synchronism with the character code and is used in driving the corresponding attribute control hardware, and thereby modifying the display data.
In the above-mentioned conventional display technique, since the output signal of each bit of the attribute code stored in the refresh memory is directly supplied to the attribute control circuit, any change in the attribute specification for only a part of the display screen requires rewriting of not only the associated attribute code but also all the remaining attribute codes in the refresh memory.
The recent trend toward enhanced resolution in the display has entailed an increase in memory capacity, which in turn has resulted in a substantial increase in the amount of processing executed by the microprocessor in rewriting the refresh memory and in a drop in the display response speed and degraded operability. These drawbacks have already reached a point where they cannot be ignored.
On the other hand, there is also a trend toward increasing the number of characters being used, such as kanji (Chinese characters) and special characters used in scientific fields. Under these circumstances, sophistication of the attribute function itself is needed to make it possible to, e.g., underline at an arbitrary raster position and raster axis. This requires provision of special control circuits such as a dedicated register and a comparator, which is not desirable from an economical point of view.